Information is generally stored by an optical disc in the form of concentric or spiral tracks sometimes referred to as information tracks. A recording and/or reproducing device rotates the optical disc while using a light beam to retrieve the information from or record the information to the optical disc. As the optical disc rotates, the light beam radially traverses the optical disc while a tracking servo loop in the recording and/or reproducing device keeps the beam of light centered on the information track, or, alternately, the track will become the information track in the case of recording information to the optical disc.
A three-beam arrangement is one common arrangement used to supply tracking signals to a tracking servo loop, which is maintaining the light beam on the current track of the optical disc. In this arrangement, a laser beam passes through a diffraction grading to form a center beam and two secondary beams. The center beam is used to read or record information on the optical disc and the two secondary beams are used for tracking the current track on which the information is being read or recorded. The two secondary beams form two spots on opposite sides of a track offset with respect to each other. A photodetector array includes a main array of four photodetector sensing the reflection of the center beam and two individual photodetectors, commonly referred to as the E and F photodetectors, sensing light from the two side beams reflected off of the optical disc.
A “search” or “seek” operation is a common operation of a recording and/or reproducing device the purpose of which is to move the light beams from the current track, i.e., the track wherein the light beam is presently positioned, to a target track. During the “search” operation, the recording and/or reproducing device typically searches for the target track on the optical disc. Achieving a search operation may require the light beams to radially move across several information tracks starting from the current track before the target track is found. Once the target address is found, the optical disc storage device can return to its normal mode of retrieving or recording information.
One method of search is to estimate based on the starting location of the light beam and the physical parameters of the movement system (e.g., mass of a carriage drive, velocity of movement of an optical head), where the target track might be located, and to then initiate commands to move the optical head which controls the light beams toward this target track. The optical head is then moved to the estimated track. The estimated track is read to determine if the track has been reached. If the target track has been overshot or undershot a further estimate is made and the optical head is again moved in a direction toward the target track. These steps are then repeated until the target track is reached. The disadvantage of this approach is that it is slow because each time a track is read a close loop tracking operation must be achieved.
Another method of search is to employ a counter that keeps an accumulated total of the number of tracks crossed as the optical head is moved radially across the disc. The optical head then moves towards the target track a number of tracks determined to be the absolute value of the starting track number subtracted from the target track number. This approach speeds up the search but it is only as effective as the accuracy of the track crossing counter. Previous counting devices have counted the total number of tracks traversed whether a track is traversed in a forward or backward direction. In the situation where an optical disc is subject to vibrations and accleration forces, the optical head may move back and forward several times from the initial track to the destination track. Thus, the total number of tracks counted will be an overestimate of the actual number of tracks traversed. An additional source of error in the count may be introduced due to eccentricity of the optical disc. An inaccurate count track slows down the search, since if the target track has been overestimated or underestimated a new search must be initiated in order to move the optical head to the target track.
In order to solve the above problems, it is desirable to find an apparatus and method for counting tracks during a search, which is accurate even in the presence of vibration, acceleration forces, eccentricity, and other sources of error.